[vlc-devel] [RFC v2 1/2] executor: introduce new executor API

[Romain Vimont]

On Wed, Sep 02, 2020 at 01:00:49PM +0200, Steve Lhomme wrote:
> > > > > > just for that. I don't want to add a separate "deactivate" function for
> > > > > > closing before _Delete(). Maybe the threads should continue to dequeue
> > > > > > the pending tasks until the queue is empty in case a runnable queues
> > > > > > another one after "closing"?
> > > > > 
> > > > > In fact, I think it is reasonable to assert that the executor is not
> > > > > closing. It is the caller responsibility to ensure that a runnable is
> > > > > not submitted after it called vlc_executor_Delete().
> > > > 
> > > > I agree, you don't want to Delete the executor while you still have
> > > > pending tasks.
> > > 
> > > That means any runnable that want to launch another runnable will need to
> > > check if the executor is not canceled.
> > 
> > Yes. But this is the same "component" which provides the run() function
> > and which calls vlc_executor_Delete().
> 
> I don't fully understand. But that doesn't sound right. That means the
> design is too tied to very narrow specific cases. (ie not chaining tasks)
> 
> > > And yet if it's not canceled it may
> > > submit a task when the executor just got canceled (by another thread). And
> > 
> > s/canceled/deleted/
> > 
> > > you will end up in this assert.
> > 
> > It is the responsibility of the caller not to submit tasks once
> > vlc_executor_Delete() is called.
> 
> But there's a race condition, as explained (twice).

Sorry, I don't see the race condition, assuming:
 - vlc_executor_Cancel() returns a bool indicating if the task has been
   removed from the pending queue, or on the contrary if it was already
   taken by an executor thread;
 - the caller is responsible, by any means (possibly setting a flag with
   proper synchronization before calling vlc_executor_Delete()), to not
   submit tasks once it called vlc_executor_Delete().

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